Economical light sources with excellent color rendition are used to illuminate interior spaces, and particularly sales floors and sales rooms, as well as display windows, where the color of objects, as it appears to the human eye, is of substantial importance. Energy-efficient light sources having a color temperature in the region of about 3000K, and less, are preferred. The desired color temperature depends, among others, on the decor and utilization of the space which is to be illuminated and, also, on the type and nature of the articles which are being illuminated. It is often impossible to predict the effects of the spectral reemission characteristics of the illuminated objects. Frequently, the perceived color cannot be predicted because it is within the psychological-physiological perception of the observer. Thus, suitability of any particular light source for any special application cannot be reliably predicted and frequently must be determined experimentally. Additionally, if the nature of the illuminated objects changes, which frequently occurs in display windows, the color temperature of the light illuminating the respective objects may have to be readjusted.
The prior art provided only one single light source in which the color temperature of the light output can be changed within certain limits, namely the incandescent lamp. Upon operation of a given incandescent lamp at different power levels, the color temperature changes; unfortunately, the light flux or light output changes much more with change in power level, and hence color temperature. An objective comparison of the effect of the changed color temperature of the light in the illuminated surroundings is very difficult because of the change in power level, entirely apart from the fact that operation of incandescent lampswith low color temperatures is highly inefficient and cannot be tolerated for economical reasons. Use of different types of light sources having different color temperatures is difficult, since the respective light sources frequently can be used only with light fixtures of different radiation characteristics. Additionally, purchasing, storing and stocking light sources having different color characteristics for use only under specific circumstances is economically unacceptable. The radiation characteristics of such different light sources, likewise, can influence the subjective impression of the perceived color reemitted or reflected from illuminated objects.
Published European Patent Application 0 504 967 A1, Derra et al, describes a circuit arrangement for pulsed operation of a sodium high-pressure lamp which can be dimmed, so that the luminous flux emitted from the lamp can be controlled. The specification notes that changing the supplied energy, and hence the light output from the lamp, also changes the color temperature, and it is stated that the inventors were surprised to find that a greater luminous flux is accompanied by a higher color temperature. The behavior of a lamp in the circuit disclosed, thus, is similar to that of an incandescent lamp under dimmer control.